To keep pace with the demand for digital content, optical data rates are being pushed toward 1 Tb/s. High speed Optical Time Division Multiplexing (OTDM) using a single wavelength is an attractive option to achieve these transmission speeds. OTDM requires fast optical switches and short pulse generators, and these elements have been implemented in various ways. Electro-optic and electroabsorptive modulators (EOMs and EAMs, respectively) can directly apply an electronic switching signal to an optical carrier. Alternatively, non-linear optical effects have been used to modulate a short clock pulse to implement an all-optical mux and demux at 160 Gb/s.
Some OTDM schemes use an interferometer as an intensity modulator by inducing a phase shift between the arms via some optical non-linear interaction (e.g., cross phase modulation). These require short (˜1 ps) optical gate pulses, typically generated by mode-locked laser diodes. These schemes are either complicated, or have not been shown to be capable of sub-ps switching speeds.
Another approach is described in U.S. Pat. No. 6,826,208, where a nonlinear transmission line (NLTL) is integrated with a pulse-forming circuit. In a nonlinear transmission line, an electronic signal experiences an amplitude-dependent propagation speed that causes an input sinusoidal wave with period T to steepen as it propagates to provide a sawtooth wave output having a sharp falling edge. The pulse forming network effectively differentiates the sawtooth wave to provide short pulses in the electrical domain. Short optical pulses can be obtained by providing these electrical pulses to the electrical input of an optical modulator. However, the pulse forming circuit required by this approach can impose a significant bandwidth limitation.
Accordingly, it would be an advance in the art to provide an approach for forming optical pulses that avoids this bandwidth limitation.